A spring reverb is a system to obtain artificial reverberation
invented during the 40's by Laurens Hammond (also inventor of the
Hammond Organ). In a Spring Reverb pan, the audio signal is coupled to
one end of the spring by a transducer (a device that can convert
electrical energy into mechanical energy or viceversa). This creates
waves that propagate through the spring in both directions. At the
other end of the spring there is another transducer that converts the
motion in the spring into an electrical signal, which is then
amplified and added to the dry sound. Most spring reverb units use
several springs together, with each spring having its own
characteristics (length, dimensions, tension, etc.) resulting in a
natural reverberation by summing several delayed sounds at fixed or
random intervals. However, with real spring reverbs the user isn't
allowed to change these characteristics. A software simulation like
Type4, on the other hand, allows you to adjust parameters like "decay"
(reverb duration), the dampening factor, the virtual spring tension
and other parameters that affect the overall timbre of the reverb
effect. Why should you want to use a spring reverb rather than a
precise and modern digital reverb? Spring reverbs have typical sonic
characteristics that, nowadays, make them desirable mostly as effects
on their own rather than simulations of an acoustic phenomena.